The present invention relates to the mass production of visually readable diffractive surface gratings, and specifically to the reproduction of holographic images in multidimensional form.
Mass reproduced holograms are commonplace, appearing on credit cards, security passes, consumer packaging, novelty items, and the like. The holograms allow viewing of images formed in reflective light at locations other than the hologram surface. This can be in the form of a three-dimensional image of an object or of a complex image having different planes.
Such holograms are made by interfering two beams of coherent light at a finite angle with each other on a photosensitive medium. One of the beams interacts with an object whose image is to be recorded, and the other is a reference beam. An image of the object is focused into or near the surface of the resulting hologram by appropriate optical elements, which may include the use of another intermediate hologram. The resulting master hologram is of the surface relief type, i.e., the image formation is stored in surface variations.
Conventional mass produced holograms are manufactured by incorporating the fragile holographic master into a rigid embossing plate in order to emboss replicas. Such a rigid master is formed in an electroless bath on the surface of the photographic master. Multiple masters are them made from this first master for use in embossing holograms with heat and pressure, or by chemical softening, into a surface deformable substrate material. The most common substrate materials into which holograms are embossed include PVC, MYLAR brand plastic film, hot stamping foil, or similar plastic sheet material. Often, such material has been coated with a thin layer of reflective material, such as aluminum, in order to result in a reflective hologram. The aluminum may be vapor deposited onto the substrate in a layer which is just thick enough to provide reflective properties.
A significant disadvantage of such conventionally produced holograms is that a multi-step operation is required to produce a hologram-bearing product. First, the hologram must be transferred onto the embossing foil, after which the foil is applied or attached to another article such as a card, to complete the manufacturing of the product. This type of process may not be cost effective for all applications in which holograms may be used, thus limiting their utility.
Another disadvantage of conventional mass produced holograms is that through the embossing process, there is a degeneration in the optical quality of the hologram from the master to the finished product. A related drawback of the embossing process is that the degree of light reflectance of the reproduced holograms is of a lower quality than that of the master. As such, high quality optical effects are not readily transferrable to a substrate.
Another use for conventional holographic technology is in the production of holographic optical elements (hereinafter referred to as H.0.E.'s), which are holograms having optical quality of a level suitable for use as lenses or mirror devices in optical systems. A single master H.0.E. may act as a complex lens or plurality of lenses. The most common method of production of H.0.E. masters involves a multi-step silver halide developing and fixing process, instead of the embossing process described above. The H.0.E. master is sealed after fixing, and is then tested to ensure its compliance with previously specified requirements of the particular optical system in which it will be employed.
A significant disadvantage of conventional H.0.E.'s is that the multi-step production process is inherently inconsistent, and as such it is difficult to mass produce H.0.E.'s using conventional methods. Thus, efficiency declines as masters which do not meet the specified requirements must be discarded. As a result, the time and cost of producing effective H.0.E.'s is significant. Also, regardless of the inefficiency of the conventional process, since H.0.E.'s must be produced individually, it physically takes a substantial amount of time to produce multiple copies of a single H.0.E.
Thus, there is a need for a method of mass producing holograms in which the hologram is integrally incorporated into the finished product in a single step operation for more cost effective manufacturing, and in order to obtain higher quality optical effects. There is also a need for a method of mass producing H.0.E.'s having consistent and repeatable high quality optical properties.